Why and how do some Indigenous communities overcome material and cultural barriers to defend their rights through litigation, while others do not? This presentation addresses that question by focusing on Argentina’s Northeastern region. It argues that historical experiences of heightened contentious politics can generate durable forms of social capital that are later repurposed to mobilize Indigenous communities in the judicial arena when legal opportunity structures become favorable. Empirically, it examines how Indigenous litigation after Argentina’s 1994 constitutional reform—which recognized the ancestral rights of Indigenous peoples—was shaped by the legacy of the Agrarian Leagues, a radical mass-based movement born in the 1950s and dismantled by military regimes. Using a difference-in-differences research design at the community-year level, the analysis finds that Indigenous communities located in areas where the Agrarian Leagues had been active were significantly more likely to litigate land rights claims after the 1994 reform. These results are complemented by a process-tracing approach based on semi-structured interviews with civic association activists, legal professionals, and Indigenous leaders. Together, the findings show how historical capacities for collective action can be retooled under improved legal conditions, thereby facilitating access to the legal system for marginalized populations.